It is sometimes desirable to treat a patient having respiratory problems with air or another gas which is warmed and humidified. The air may be room air delivered under pressure by a blower or air delivered by a positive pressure ventilator. Typically, pressurized air is heated and humidified and then delivered through a hose and either a cannula or a mask to the patient's respiratory tract. For some treatments, it is desirable to saturate the air with water vapor. In some cases, the air also may be enriched with oxygen prior to humidification.
When a heated gas is humidified to a high relative humidity, there are problems in delivering the gas to the patient. If the humidified gas cools as it is delivered to the patient, a portion of the vapor will condense on the walls of the delivery hose. Consequently, the gas will not be delivered to the patient at the desired temperature and with the desired vapor content. It is known, for example, from
U.S. Pat. No. 3,638,926, that the delivery hose may have an embedded helical electric heater wire for heating the gas as it is delivered. However, it is difficult to accurately control the heating wire to maintain the temperature and saturation level of the gas constant as the gas flows through the hose. Condensation may occur at any cooler point on the hose.
It is also known that two coaxial hoses may be used for delivering to a patient a gas containing an anesthetic vapor. As is taught in U.S. Pat. No. 4,637,384, the gas and anesthetic vapor are delivered through an inner one of the hoses. Exhaled gases flow through an outer annular passage which surrounds the inner hose to carry such gases away from the patient. It does not appear that the delivered gas and anesthetic vapors are heated and the exhaled gases are not used to maintain the temperature of the gas and anesthetic vapors being delivered to the patient.
U.S. Pat. No. 3,616,796 teaches the delivery of heated humidified gas to a patient using two coaxial hoses. The gas is delivered through the inner one of the hoses which is water permeable. Heated water is passed through the outer annular passage. The heated water heats the delivered gas and water which permeates the inner tube humidifies the gas. This system has no control over the level that the gas is humidified.